1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device. In particular, the invention relates to complementary phase shift mask (c:PSM) imaging in lithography.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Imaging Enhancement Techniques (IET) are commonly used by IC manufacturers to push the technical boundaries of the lithographic process to their limits in order to print smaller and smaller features onto the substrate, at or beyond the resolution limit of the lithographic apparatus being used. One form of IET is the Complementary Phase Shift Mask (c:PSM) technique. In c:PSM two exposure steps are employed. In the first exposure a dark field alternating Phase Shift Mask is used to define the gates of the IC to be printed. (The gates are typically dark lines printed on a bright background and connected to larger structures. Gates occur both as densely packed as well as isolated lines.) In the second exposure a binary “trim mask” is used to remove unwanted residual photoresist from the substrate. The trim mask is also used to print features with sizes much larger than the critical dimension (CD) of the gates. The CD is a well-understood term in the art, referring in general to the dimensions of the smallest geometrical features (e.g. width of gate lines, contacts, trenches etc.) which can be formed during semiconductor device/circuit manufacturing using given technology. (The CD is also sometimes referred to as the linewidth or feature width and the term may sometimes simply be used to refer to the width of a particular type of feature, for example gate lines, printed in resist, measured at a specific height above the substrate.) The exposure order of these two exposure steps may alternatively be reversed but for clarity the above-mentioned exposure sequence is maintained throughout this text. Summarized descriptions of the c:PSM technique appear in the following prior art documents:                ‘Resolution Enhancement Techniques in Optical Lithography’, Alfred K. Wong, SPIE press, March 2001;        ‘Performance optimization of the Double Exposure Alt PSM for sub-100 nm ICs’, G. Vandenberghe, BACUS 4562-43.        
One disadvantage of the c:PSM technique is that current trim masks may have a negative impact on imaging performance. In particular, the isofocal CD increases significantly (compared with similar processes where the trim step is not used) such that it may not be possible to achieve the required CD uniformity for the gate lines. The CD uniformity, sometimes alternatively referred to as “CD control”, is a well understood term in the art, referring to the uniformity of the CD (of a particular type of feature) across an exposure field, a wafer or multiple wafers.